Global positioning systems (GPS) are widely used for navigating ships and aircraft.
However, while these systems give accurate position information, they do not by themselves give accurate information about the orientation of a platform, also known as its attitude. Inertial navigation systems (INS) mitigate GPS deficiencies; however, these inertial navigation systems are not accurate over long time periods. Errors may accumulate at rates of about an arc-second per hour to an arc-minute per hour. Periodic alignment of the inertial navigation systems is required using an external reference system such as a GPS system. In addition, inertial navigation systems can be very expensive, depending on the required accuracy. A less expensive way to measure platform attitude is by using tilt sensors (to measure local horizontal) and magnetometers (to measure the direction of the earth's local magnetic field). However, a magnetometer will not work in all locations, and its accuracy is limited by changes in the earth's magnetic field. The accuracy of a tilt sensor is limited by local deviations of the gravity vector from the direction to the center of the earth caused by geological formations. In addition, a tilt sensor may not work well on a moving platform which is accelerating relative to the earth.
For centuries navigators have used the sky for the most fundamental and accurate inertial system available, in which each star is a benchmark. Cataloged positions and motions of the stars define the celestial reference frame. The problem is stars are hard to see during the daytime. Efforts have been made to navigate by stars during daytime using very sensitive visible light charge coupled device (CCD) cameras, but these efforts as far as we know, have been unsuccessful due to the very limited number of stars that can be seen with this sensor.
A need exists for a platform attitude determination system which can work in daytime and nighttime without requiring an expensive inertial navigation system, and which can provide accurate attitude information on an accelerating platform.